In signal transmission technology, PDS (Passive Double Star) structure has been studied and researched to apply to ATM (Asynchronous Transfer Mode) network services. This kind of technology is described in literatures, "Time-Slot Distributing Method for ATM-PDS", by Nakajima and Takigawa, 1993 Electronic Information Transmission Society, Autumn Conference B-669, 1993; and "ATM Based Passive Double Star System Offering B-ISDN, N-ISDN and POTS" by Y. Takizawa et al., GLOBCOM '93, pp-14-18, 1993.
An optical communication system with ATM-PDS structure includes a plurality of ONUs (Optical Network Units), which are placed in subscribers' premises, an SLT (Subscriber Line Terminal) and a star couple connecting the ONUs and the SLT to each other using optical fibers. The SLT includes a plurality of OSUs (Optical Subscriber Unit) each operating as interface to the corresponding ONUs. An optical signal transmitted from the OSU is branched by the star coupler and the branched signals are transmitted to the ONUs in a downward direction. On the other hand, the star coupler multiplies optical signals from the ONUs and the multiplied signal is transmitted to the OSU in an upward direction. If the ONUs transmit optical signals independently in the upward direction, those signals are competed at the star coupler. For that reason, the OSU transmits a permission signal in the downward direction, which specifies one ONU, to allow only the specified ONU to transmits an optical signal in the upward direction.
In practice, an optical signal in the downward direction is transmitted cell by cell, which is ATM-PDS cell of sixty bites. The OSU adds at the first two bytes in the ATM-PDS cell an identification number specifying one ONU which is allowed to transmit an optical signal. When the specified ONU receives an ATM-PDS cell including its identification number, the ONU transmits a cell in the upward direction within a predetermined period of time. In ATM-PDS structure, the same width of transmission band is not always distributed to the ONUs. In other words, ONU identification numbers to be added in ATM-PDS cells in the downward direction are not equally distributed to the ONUs, but must be distributed in accordance with subscribers' contracts, respectively.
The OSU is provided with a band control circuit to control the distribution of transmission bands to the ONUs. A conventional band control circuit includes a plurality of transmission timing controllers, each operating for the corresponding ONU, and a competition controller, which arbitrates competing demand of transmission among the ONUs. Each of the transmission timing controller includes cell interval counter and a waiting time counter. Each of the cell interval counter is supplied with a specified cell interval of the corresponding ONU. Each cell interval counter is reset when a permission signal is supplied to the corresponding ONU, and perform increment operation. When the counted value of the interval counters reach the specified cell intervals, the counted values are maintained until a reset signal is supplied thereto. Each waiting time counter starts increment operation when the corresponding cell interval counter reaches the specified cell interval. The counted value of each waiting time counter represents a waiting time since the corresponding cell interval counter reaches the specified cell interval.
The competition controller selects one from the ONUs, whose waiting time is the longest which means the corresponding waiting counter stores the largest number, and transmits the identification number of the selected ONU. The identification number is supplied as a reset signal to the cell interval counter and the waiting time counter for the selected ONU.
According to conventional band control circuits, however, there is a disadvantage in that some ONUs can not transmit signals within the specified cell intervals thereof. The delay time may be a maximum of cell intervals corresponding to the number of ONUs connected to the OSU. In addition, none of the ONUs transmits an optical signal at a cell interval that is shorter than its specified cell interval. As a result, the contracted bands may not be distributed to the subscribers. In other words, the average of distributed cell interval for the ONU may be longer than the contracted cell interval.